The present invention relates to thermistors employed for temperature sensing in a control system of the type where periodic measurements are taken of the change in resistance of the thermistor for determining changes in the temperature of the medium to which the thermistor is exposed. In such applications, it has been found convenient to provide a low level of current through the thermistor to provide some heating of the thermistor for reference purposes, and to monitor the change in resistance of the thermistor from the self-heated level which are caused by changes in temperature of the surrounding medium.
Self-heated thermistors have been found to be particularly suitable for measuring changes in temperature in refrigerants circulated within an air conditioning or refrigeration system, such as that employed for cooling the passenger compartment of a motorized vehicle. Such a system is shown and described in U.S. Pat. No. 4,993,231, commonly owned by the Assignee of the present invention. In manufacturing such known control systems for automotive passenger compartment air conditioners, it has been discovered that commercially available thermistors typically have resistance versus temperature properties which deviate significantly from the nominal values thereof provided by the manufacturer of the thermistor. This deviation in resistance versus temperature properties can result in a substantial shift or skewing of the operation or response of the control system in view of the different value of resistance measured for any given temperature. Error in the measured resistance can result in a different response from the controller than is desired where the controller is programmed to respond in accordance with a particular algorithm or value from a look-up table. Where air conditioning systems for vehicles are assembled in mass production, a significant variation in resistance versus temperature properties of the commercially available thermistors can result in a prohibitively wide variation in the response of the air conditioner control system sufficient to render the control system effectively dysfunctional. The above-mentioned problem of employing self-heated thermistors as temperature sensors in vehicle air conditioning systems has been found to be particularly troublesome where the thermistor is disposed to sense refrigerant temperature, and to provide an electrical temperature signal to a microprocessor-based controller for effecting control functions in the system, such as cycling the condenser fan or compressor clutch in systems having a mechanical thermal expansion valve. Where the thermistor temperature signal is employed by a microprocessor based controller to control the operation of an electrically operated thermal expansion valve error in the conversion of the measured resistance to temperature can result in inability to adequately control refrigerant flow to the evaporator.
It has therefore long been desired to find a way or means of compensating for the deviation from nominal value of temperature versus resistance of a thermistor employed for periodic measurement of temperature in a medium to be controlled by a control system receiving a signal from the thermistor. It has been particularly desired to find a technique for compensating for such variation in a thermistor employed for periodic temperature measurements in a refrigeration system.